Natural gas is a fossil fuel. Like other fossil fuels such as coal and oil, natural gas forms from the plants, animals, and microorganisms that lived millions of years ago.

There are several different theories to explain how fossil fuels are formed. The most prevalent theory is that they form underground, under intense conditions. As plants, animals, and microorganisms decompose, they are gradually covered by layers of soil, sediment, and sometimes rock. Over millions of years, the organic matter is compressed. As the organic matter moves deeper into Earth’s crust, it encounters higher and higher temperatures.

The combination of compression and high temperature causes the carbon bonds in the organic matter to break down. This molecular breakdown produces thermogenic methane—natural gas. Methane, probably the most abundant organic compound on Earth, is made of carbon and hydrogen (CH4).

Natural gas deposits are often found near oil deposits. Deposits of natural gas close to Earth’s surface are usually dwarfed by nearby oil deposits. Deeper deposits—formed at higher temperatures and under more pressure—have more natural gas than oil. The deepest deposits can be made up of pure natural gas.

Natural gas does not have to be formed deep underground, however. It can also be formed by tiny microorganisms called methanogens. Methanogens live in the intestines of animals (including humans) and in low-oxygen areas near the surface of Earth. Landfills, for example, are full of decomposing matter that methanogens break down into a type of methane called biogenic methane. The process of methanogens creating natural gas (methane) is called methanogenesis.

Although most biogenic methane escapes into the atmosphere, there are new technologies being created to contain and harvest this potential energy source.

Thermogenic methane—the natural gas formed deep beneath Earth’s surface—can also escape into the atmosphere. Some of the gas is able to rise through permeable matter, such as porous rock, and eventually dissipate into the atmosphere.

However, most thermogenic methane that rises toward the surface encounters geological formations that are too impermeable for it to escape. These rock formations are called sedimentary basins.

Sedimentary basins trap huge reservoirs of natural gas. In order to gain access to these natural gas reservoirs, a hole (sometimes called a well) must be drilled through the rock to allow the gas to escape and be harvested.

Sedimentary basins rich in natural gas are found all over the world. The deserts of Saudi Arabia, the humid tropics of Venezuela, and the freezing Arctic of the U.S. state of Alaska are all sources of natural gas. Outside Alaska, U.S. basins are primarily around the states bordering the Gulf of Mexico, including Texas and Louisiana. Recently, the northern states of North Dakota, South Dakota, and Montana have developed significant drilling facilities in sedimentary basins.

Types of Natural Gas

Natural gas that is economical to extract and easily accessible is considered “conventional.” Conventional gas is trapped in permeable material beneath impermeable rock.

Natural gas found in other geological settings is not always so easy or practical to extract. This gas is called “unco